The present invention relates to a concentric rotary valve (one in which the rotatable stem, support bearings, disc, and seats have a common axis) having a rotatable stem of a valve disc supported by bearing portions and metal seats mounted in a valve housing to contact metal sheets on the valve disc.
As shown in FIGS. 9 and 10 of the accompanying drawings, the rotatable stem 100 and the bearing portions 101 of a conventional rotary valve both have a circular cross section. It is difficult in manufacture to place the rotational axis of seat surfaces in precise alignment with the axis of the rotatable stem. Such an axial displacement causes an irregular distribution of forces between the seat surfaces and the bearing portions and ill-balanced contacting conditions of the seat surfaces, which result in fluid leakage. While rubber seats have a nip margin of 0.5 to 1 mm allowing for a 0.05 to 0.1 mm displacement of the axes, metal seats even in a large valve have a nip margine of no more than 0.05 to 0.1 mm and have to be assembled with an extremely strict axial alignment permitting a displacement as small as 0.005 to 0.01 mm. Because of this manufacturing problem a concentric rotary valve providing a tight contact between metal seats has been given up as impossible particularly where no slipperiness is allowed on the seat surfaces. There still is a technical problem with the products that have overcome the above manufacturing problem. This is explained below with reference to an inclined cylinder type concentric butterfly valve, which is regarded as the most highly developed concentric butterfly valve with metal seats, as mounted in a medium or high pressure fluid pipeline.
As a fluid pressure acts on a valve disc 102 in its closed position as shown in FIG. 9, the presence of a clearance 103 permits the valve disc 102 and the rotatable stem 100 to shift along the direction of the fluid pressure. At this time, the valve disc 102 still receives a torque acting in a direction to open it, as shown in FIG. 10. Thus the valve disc 102 keeps rotating while following a deformation of the valve housing and presses against metal seats 104 on the housing. Since the metal seat 104 disposed on the lefthand side of the valve housing in FIG. 10 is inclined so as to constrict in the direction in which the fluid pressure acts, an arcuate flexion of the valve disc 102 added to its initial shift noted above reinforces an intimate contact between the seat on the lefthand side of the valve disc and the seat 104 on the valve housing. Conversely, the metal seat 104 disposed on the righthand side of the valve housing in FIG. 10 is inclined so as to broaden in the direction in which the fluid pressure is acting, and therefore the arcuate flexion of the valve disc 102 relaxes the contact between the seat on the righthand side of the valve disc and the seat 104 on the righthand side of the housing, which results in fluid leakage.